Integrated waveguide optical isolator

Zhang, Cui (2017) Integrated waveguide optical isolator. PhD thesis, University of Glasgow.

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This project is aimed at the integration of a polarisation-insensitive optical waveguide isolator on a Silicon-on-Insulator platform. The final device must provide comparable performance for both Transverse Electric and Transverse Magnetic modes at a wavelength of 1550 nm. This is achieved through two core components, a 45° Nonreciprocal Polarisation Mode Converter, and a 45° Reciprocal Polarisation Mode Converter.
In order to realise the Nonreciprocal Polarisation Mode Converter, several materials were investigated, all consisting of Silicon-on-insulator substrates with various thicknesses of the core layer that were coated with films of Magneto-Optic garnet materials. A wide number of Magneto-Optic garnet materials were tested. Among them, the Cerium-Terbium Iron Garnet proved the most promising for two reasons: first, it has a considerable Faraday rotation coefficient; secondly, it can be grown in crystalline form without the need for a buffer/seed layer, necessary for growing most other garnets.
Simulations were carried out for all grown materials in order to identify the most promising design. The simulated designs, however, could not always be translated into fabricated devices, as sometimes growth challenges would hinder the quality of the material. Since the growths on the 340 nm and 500 nm Silicon-on-Insulator platforms provided the best material quality, devices on these material systems were fabricated and optically characterised.
Nonreciprocal isolation performance was observed in all fabricated devices, independently of the Magneto-Optic garnet used. On the 340 nm Silicon-on-Insulator platform, the best performance was obtained when Bismuth-Terbium Iron Garnet, either on its own or in combination with Terbium Iron Garnet, was used as Magneto-Optic periodic cladding, leading to more than 3/4π Stokes vector angle. On the 500 nm Silicon-on-Insulator platform instead, Cerium-Yttrium Iron Garnet, either by Magnesium Oxide or on Yttrium Iron Garnet, provided a calculated isolation ratio of 11.6 dB. The length of the fabricated devices ranged between 3 mm and 6 mm.
A reproducible device fabrication process, optical characterisation method and dedicated data analysis process had to be developed for this project. Nonreciprocal Polarisation Mode Conversion was demonstrated for devices on both the 500 nm and 340 nm Silicon-on-Insulator platforms. Moreover, in order to achieve integration of Magneto-Optic garnet materials on Silicon-on-Insulator substrates, Radio-Frequency sputtering was preferred to wafer bonding as it improves the controllability and lends itself better to scaling up production.
With regard to the Reciprocal Polarisation Mode Converter, an asymmetric structure consisting of an L-shaped waveguide was chosen. In such a structure, the rotation of the optical axis enables an injected linear polarisation mode to excite hybrid modes and reciprocal mode conversion. The research carried out in this project for the reciprocal polarisation mode converter helped identify major issues with fabrication and characterisation, and lead to the proposal of a new design for further research.
This work successfully realised the first integrated polarisation-independent Faraday rotator showing comparable performance for both Transverse Electric and Transverse Magnetic modes. Device operation was based on nonreciprocal polarisation mode conversion, and it was demonstrated on both 500 nm and 340 nm Silicon-on-Insulator platforms. The results shown in this work in terms of performance and footprint prove the technology is suitable for optical integration.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Colleges/Schools: College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering
Funder's Name: Engineering & Physical Sciences Research Council (EPSRC)
Supervisor's Name: Hutchings, Professor David
Date of Award: 2017
Depositing User: MISS CUI ZHANG
Unique ID: glathesis:2017-7994
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 18 Apr 2017 09:42
Last Modified: 30 May 2017 07:49

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